1. Field of the Invention
The present invention relates to an oxide semiconductor element and a semiconductor device.
2. Description of the Related Art
Semiconductor elements (hereinafter referred to as silicon semiconductor elements) such as transistors using silicon for semiconductor layers are used for a variety of semiconductor devices and have become essential technologies for manufacturing semiconductor devices. In order to manufacture large semiconductor devices, a method using a material which is suitable for increasing in size, for example, glass or the like, for a substrate, and thin-film silicon for a semiconductor layer, which can be formed over a large area has been widely employed.
In such semiconductor elements using thin-film silicon, semiconductor layers need to be formed at temperatures less than or equal to the upper temperature limits of substrates. Thus, amorphous silicon and polysilicon which can be formed at relatively low temperatures are widely used.
Amorphous silicon has advantages of being able to be deposited over a large area and allowing semiconductor elements having uniform element characteristics to be manufactured by a simple process at relatively low cost; thus, amorphous silicon has been widely used for semiconductor devices with a large area, such as solar batteries. Meanwhile, amorphous silicon has a disadvantage of low electron mobility due to the fact that an amorphous structure causes a scattering of electrons at grain boundaries.
In order to make up for the disadvantage, polysilicon, whose mobility is improved by irradiating amorphous silicon with laser or the like to be locally dissolved and recrystallized, or by crystallization using a catalytic element, is widely used in semiconductor devices such as liquid crystal displays in which both large area and high carrier mobility need to be achieved.
In addition, in recent years, oxide semiconductors that are metal oxides having semiconductor characteristics have attracted attention as novel semiconductor layer materials having high mobility, which is an advantage of polysilicon, and a uniform element characteristic, which is an advantage of amorphous silicon.
As semiconductor elements (hereinafter referred to as oxide semiconductor elements) such as transistors using oxide semiconductors as semiconductor layers, for example, as in Patent Documents 1 and 2, a thin film transistor manufactured using tin oxide, indium oxide, zinc oxide, or the like has been proposed.
As described above, oxide semiconductor elements have a variety of advantages. However, since a phenomenon in which the threshold voltage is changed due to light irradiation occurs, there is a problem in reliability. In recent years, it has been discussed that oxygen deficiency or hydrogen in oxide semiconductor layers affects a change in threshold voltage by light irradiation.